Joan P Marler

Low energy positron scattering and annihilation studies using a high resolution trap-based beam

Abstract A high resolution positron beam, generated from a Penning–Malmberg trap, has been used for a range of low energy scattering and annihilation studies on atoms and molecules. We describe measurements of total scattering, differential elastic scattering and integral vibrational and electronic excitation cross sections for a number of atoms and molecules using this beam, and compare the absolute cross sections that are obtained with data from electron impact. The first study of annihilation on atoms and molecules as a function of positron energy is described. The results in molecules indicate large resonant enhancements of the annihilation rates at energies corresponding to those of the molecular vibrations.

Positioning of the rf potential minimum line of a linear Paul trap with micrometer precision

We demonstrate a general technique to achieve a precise radial displacement of the nodal line of the radiofrequency (rf) field in a linear Paul trap. The technique relies on the selective adjustment of the load capacitance of the trap electrodes, achieved through the addition of capacitors to the basic resonant rf circuit used to drive the trap. Displacements of up to ~100 µm with micrometer precision are measured using a combination of fluorescence images of ion Coulomb crystals and coherent coupling of such crystals to a mode of an optical cavity. The displacements are made without measurable distortion of the shape or structure of the Coulomb crystals, as well as without introducing excess heating commonly associated with the radial displacement of crystals by adjustment through static potentials. We expect this technique to be of importance for future developments of microtrap architectures and ion-based cavity QED.

Large ion Coulomb crystals: A near-ideal medium for coupling optical cavity modes to matter

We present an investigation of the coherent coupling of various transverse field modes of an optical cavity to ion Coulomb crystals. The obtained experimental results, which include the demonstration of identical collective coupling rates for different transverse modes of a cavity field to ions in the same large Coulomb crystal, are in excellent agreement with theoretical predictions. The results furthermore suggest that Coulomb crystals in the future may serve as near-ideal media for high-fidelity multimode quantum information processing and communication purposes, including the generation and storage of single-photon qubits encoded in different transverse modes.

Note: High density pulsed molecular beam for cold ion chemistry

A recent expansion of cold and ultracold molecule applications has led to renewed focus on molecular species preparation under ultrahigh vacuum conditions. Meanwhile, molecular beams have been used to study gas phase chemical reactions for decades. In this paper, we describe an apparatus that uses pulsed molecular beam technology to achieve high local gas densities, leading to faster reaction rates with cold trapped ions. We characterize the beams spatial profile using the trapped ions themselves. This apparatus could be used for preparation of molecular species by reactions requiring excitation of trapped ion precursors to states with short lifetimes or for obtaining a high reaction rate with minimal increase of background chamber pressure.

Kinetic energy offsets for multicharged ions from an electron beam ion source

Using a retarding field analyzer, we have measured offsets between the nominal and measured kinetic energy of multicharged ions extracted from an electron beam ion source (EBIS). By varying source parameters, a shift in ion kinetic energy was attributed to the trapping potential produced by the space charge of the electron beam within the EBIS. The space charge of the electron beam depends on its charge density, which in turn depends on the amount of negative charge (electron beam current) and its velocity (electron beam energy). The electron beam current and electron beam energy were both varied to obtain electron beams of varying space charge and these were related to the observed kinetic energy offsets for Ar4+ and Ar8+ ion beams. Knowledge of these offsets is important for studies that seek to utilize slow, i.e., low kinetic energy, multicharged ions to exploit their high potential energies for processes such as surface modification. In addition, we show that these offsets can be utilized to estimate the effective radius of the electron beam inside the trap.

Trapped ion chain thermometry and mass spectrometry through imaging.

We demonstrate a spatial-imaging thermometry technique for ions in a one-dimensional Coulomb crystal by relating their imaged spatial extent along the linear radiofrequency ion trap axis to normal modes of vibration of coupled oscillators in a harmonic potential. We also use the thermal spatial spread of “bright” ions in the case of a two-species mixed chain to measure the center-of-mass resonance frequency of the entire chain and infer the molecular composition of the co-trapped “dark” ions. These non-destructive techniques create new possibilities for better understanding of sympathetic cooling in mixed-ion chains, improving few-ion mass spectrometry, and trapped-ion thermometry without requiring a scan of Doppler cooling parameters.

A gas cell apparatus for measuring charge exchange cross sections with multicharged ions

A gas cell apparatus to measure charge exchange cross sections for charge state- and energy-resolved ion beams with neutrals is described. The design features a short well-defined interaction region required for beams of multicharged ions with high cross sections. Our method includes measuring the beam transmission at four different neutral pressures and extracting the cross section from the slope of a beam loss vs pressure plot. The design and procedure were tested for Ar+ interacting with neutral Ar gas over the incident ion energy range of 1.0-5.0 keV. The charge exchange cross sections agree well with previous complementary measurement techniques.

Highly charged ion beams and their applications

While much previous work with highly charged ions has been performed with the ions in the plasma state in which they were formed, beams of highly charged ions hold promise for exciting new experiments. Specifically low energy beams with a high degree of charge state purity are a prerequisite for momentum resolved cross section measurements and for efficient loading of highly charged ions into UHV traps for spectroscopy. The Clemson University facility is optimized for the delivery of such beams of highly charged ions with low kinetic energies. Near term experiments include energy resolved charge exchange with neutral targets.

Collective strong coupling with ion Coulomb crystals in an optical cavity

Cavity Quantum Electrodynamics (CQED) is an attractive platform for the realization of efficient light-matter quantum interfaces at the single photon level. Such interfaces are key elements for the development of quantum information and communication science due to the need for swapping information between flying and stationary qubits. Here, we report on the first CQED demonstration of collective strong coupling between atomic ions in a solid in the form of an ion Coulomb crystal and an optical field [1].

Low-Energy Positron-Matter Interactions Using Trap-Based Beams

We present an overview of nonneutral plasma techniques developed to study the interaction of low-energy positrons with atoms and molecules. Both scattering and positron annihilation experiments are described. The scattering experiments provide the first state-resolved cross sections for both vibrational excitation of molecules and electronic excitation of atoms and molecules by positron impact. The annihilation experiments provide the first energy-resolved measurements of positron annihilation. Extensions of these techniques are briefly discussed, including work to create a new generation of positron beams with millielectron volt energy resolution and the development of methods to study atomic clusters and dust grains.